Generally, there are two types of guide systems for controlling a transverse position of a moving web. A first type of guide system for controlling a transverse position of a moving web is a passive system. An example of a passive system is a crowned roller, also called a convex roller, having a greater radius in the center than at the edges. Crowned rollers are effective at controlling webs that are relatively thick in relation to the width of the web such as sanding belts and conveyor belts. Another passive type of guide system is a tapered roller with a flange. The taper on the roller directs the web towards the flange. The web edge contacts the flange and thereby controls the transverse position of the web. A tapered roller with a flange is commonly used to control the lateral position of a narrow web, such as a videotape.
However, a passive guide system cannot guide wide, thin webs because, depending on the type of passive guide system, either the edge of the web tends to buckle or the web tends to develop wrinkles. To effectively control a wide, thin web an active guide system is required.
A typical active guide system includes a sensing device for locating the position of the web, a mechanical positioning device, a control system for determining an error from a desired transverse location and an actuator that receives a signal from the control system and manipulates the mechanical positioning device. A typical control system used for actively guiding a thin, wide web is a closed loop feedback control system.
Typically, a web to be processed has been previously wound into a roll. During the winding process, the web is not perfectly wound and typically has transverse positioning errors in the form of a zigzag or a weave. When the web is unwound, the zigzag or weave errors recur causing transverse web positioning problems.
It is known to control a moving web in relation to a selected transverse position by positioning a first positioning guide proximate a second positioning guide, then passing the web through the first positioning guide to reduce angular and transverse position errors. The web is then passed through the second positioning guide where the second positioning guide positions the moving web independently of the first positioning guide with a mechanism having zero-backlash.
The transverse location of the moving web is sensed at the second positioning guide with a sensor and the transverse location of the web at the second positioning guide is transmitted to a controller.
The controller then manipulates a zero-backlash actuator so as to control the transverse position of the web.